1. Field of the Invention
The present invention relates to an implantable medical device, a communication method used in an implantable medical system, and an implantable medical system.
2. Description of the Prior Art
In RF coupled systems, which are perhaps the most commonly employed communication systems in modern implantable device systems, information is transferred from a transmitting coil to a receiving coil via a radio-frequency carrier signal. The carrier signal is modulated with the data that are to be transmitted using an appropriate modulation scheme, such as phase shift keying (PSK), frequency shift keying (FSK), or pulse position modulation (PPM), among numerous others. The modulated carrier induces a voltage in the receiving coil that tracks the modulated carrier signal. This received signal is then demodulated in order to recover the transmitted data. Because the stainless steel or titanium can commonly used to hermetically enclose an implanted device acts as a low-pass filter for the transmitted RF signals, attenuation increases as frequency is increased. Devices currently on the market have a maximum frequency of less than 200 kHz. Also, the transmitting range has been limited to 50 to 100 mm or so.
Depending upon the type of modulation and demodulation used in an RF communication system, the data or bit rate cannot exceed a predetermined fraction of the carrier frequency; otherwise, the ability to reliably distinguish between modulation representing a digital (binary) xe2x80x9c1xe2x80x9d from a digital xe2x80x9c0xe2x80x9d is compromised. Techniques are known which encode digital data to transmit more data per unit time and reduce implanted device current drain. However, at very high data transmission rates, the current drain would be very high.
RF communication programming units typically interface with the implanted device through the use of a programming head or programming paddle, a handheld unit adapted to be placed on the patient""s body over the implant site of the patient""s implanted device. In some cases, a magnet in the programming head effects closure of a reed switch in the implanted device to initiate a communication session (this is a safeguard against accidental programming of the device; otherwise, closure of the reed switch has little meaning as far as communication of information). Thereafter, uplink and downlink communication takes place between the implanted device""s transmitter and receiver and a receiver and transmitter disposed within the programming head.
An implanted medical device, IMD, that utilizes so-called long range telemetry for bi-directional communication with an external telemetry device may suffer from too frequent, and also unwanted, activation of the telemetry receiver. Long range telemetry is herein defined as telemetry performed at a maximal distance between the implanted device and the sending unit of the external device of approximately 0.5 m up to 2 m.
The IMD receiver electronics must periodically, or continuously, xe2x80x9clistenxe2x80x9d for attempts made from the surrounding environment to establish contact over the communication channel. In order to be able to communicate over longer distances, the IMD receiver must be made rather sensitive. One consequence of making the IMD receiver rather sensitive is that it will thus respond to signals not necessarily intended for the implanted device. Nevertheless the electronics must be alert, decode the signals or noise and cannot revert to an idle state until it has made the decision whether the received signal was aimed for the device or not. Activating the entire chain of the receiver electronics now and then significantly increases the battery drain.
U.S. Pat. No. 5,683,432 discloses an adaptive, performance-optimizing communication system for communicating with an implanted medical device. In the system signals are transmitted and received in accordance with predetermined, interrelated operational parameters, such as transmission rate, transmitter power, and the like. Various aspects of system performance, including bit error rate in received signals, the strength of received signals, the signal-to-noise ratio of received signals, the presence of local RF noise and non-telemetry related RF signals, and the like, are dynamically monitored by the communication system, to determine whether predetermined system performance goals are being met. If it is determined that one or more system performance goals are not being met, one or more operational parameters may be automatically adjusted so that desired performance can be achieved.
U.S. Pat. No. 6,201,993 discloses a medical device for detecting an RF signal transmitted between an implantable medical device and an external medical device programmer in a telemetry session. In order to avoid transient and steady state noise in the RF signal transmitted uplink, i.e. from the implantable medical device to the external device, the receiver section of the external device is provided with an adaptive comparator circuit for comparing a demodulated uplink signal amplitude with an adaptive threshold signal. A receiver output signal is generated when the demodulated uplink signal amplitude exceeds the adaptive threshold amplitude of the adaptive threshold signal.
There are two main situations where long range telemetry is particularly useful.
The first situation is during implantation of the medical device where it is difficult to perform telemetry due to the requirements of keeping a sterile environment close to the patient.
The second situation is when a patient performs a follow up at home having the external device communicating to the physician e.g. via the telephone line. It is sometimes considered cumbersome to have to hold the telemetry head close to the skin. If instead it were possible to e.g. sit on a sofa up to e.g. 2 meters from the telemetry head the follow up would be much easier to perform.
None of the above discussed known techniques are directed to any of the problems related to long range telemetry.
An object of the present invention is to be able to control the remote accessibility of an implanted device and prevent external units from starting a bi-directional communication session without letting the IMD wearer being aware.
Further objects of the present invention are to reduce the power consumption caused by overly frequent receiver activation as well as to reduce unintentional access of an implanted device, especially when performing long range telemetry.
The above objects are achieved in accordance with the principles of the present invention in an implantable medical device, and a method for operating such a device, wherein the implantable medical device includes circuitry that requires activation by an activation signal in order to operate, and wherein a telecommunication unit is provided for bi-directional communication with an external device, the telecommunication unit including a receiver which receives communication signals, including an activation signal, from the external device, and having a discriminator, with a variable discriminator threshold, the discriminator allowing the activation signal to proceed to the receiver only if the signal strength of the activation signal exceeds the discriminator threshold, and wherein a control unit is provided that is connected to the discriminator, the control unit emitting control signals to the discriminator to set the discriminator threshold to a selected level.
An advantage obtained by the present invention is the increased communication safety achieved by making the discriminator threshold adjustable. Although the lowest threshold level is denoted xe2x80x9clong range thresholdxe2x80x9d in practice the communication is often performed by having the external transmitting means arranged close to the skin of the patient. However, by using the present invention a higher safety is achieved when performing communication at a close distance. At the same time the communication system is quite insensitive to situations that intentionally or unintentionally may occur if the sending antenna of the external device is moved a bit farther away from the patient.
A further advantage of the present invention is it reduces the battery drain because the entire receiver electronics need not be activated during periods of noise.
In theory, of course it is possible that an IMD communication channel may be opened by simply increasing the transmitter energy from the external device, regardless of the distance between the IMD and the external device. However, in practice, the available frequency bands granted by the telecommunication authorities are restricted in use with respect to the emitted power within the bands.